1. Field of the Invention
The present invention relates to an acceleration sensor using a piezoelectric substrate, and particularly to a small-sized and high-sensitivity acceleration sensor.
2. Description of Related Art
Acceleration sensors have been conventionally used for applications such as detection of shock applied from the exterior to electronic equipment such as hard disk drives.
For example, acceleration sensors of such a type that charge detection electrodes are arranged on both main surfaces of a piezoelectric substrate in a rectangular parallelepiped shape to produce a vibrating element, and its end is supported by a supporting member have been known (see JP 2000-321299 A, and JP 7-36064 U, for example).
In such an acceleration sensor, the vibrating element is deflected by applied acceleration so that the piezoelectric substrate is distorted, and charges are generated in the charge detection electrodes formed on both the main surfaces of the piezoelectric substrate by a piezoelectric effect.
The acceleration is detected by the charges or a voltage generated between the charge detection electrodes by the charges.
Which of the charges and the voltage should be utilized to detect the acceleration differs depending on an acceleration detection device that detects acceleration utilizing the acceleration sensor. Generally, acceleration detection sensitivity in a case where the acceleration is detected by the generated charges is referred to as “charge sensitivity”, and acceleration detection sensitivity in a case where the acceleration is detected by the generated voltage is referred to as “voltage sensitivity”. It is desirable that the charge sensitivity and the voltage sensitivity are higher as the acceleration sensor.
For example, an output voltage V in a case where a force F produced by acceleration is applied to an acceleration sensor in which one end in the longitudinal direction of a bimorph-type vibrating element obtained by affixing two piezoelectric substrates in a rectangular parallelepiped shape to each other in the thickness direction is as follows, letting D be the piezoelectric constant of the piezoelectric substrate, L be the length of a free vibration region, which is not held by the supporting member, in the vibrating element, letting W be the width of the vibrating element, and T be the thickness of the vibrating element:V=(3/2)·D·L·F/(W·T)The output voltage V is proportional to the length L of the free vibration region in the vibrating element, and is inversely proportional to the width W and the thickness T of the vibrating element.
Although the length L of the free vibration region in the vibrating element must be increased to reduce the width W and the thickness T of the vibrating element in order to improve the acceleration detection sensitivity of the acceleration sensor, therefore, the increase in the length L of the free vibration region in the vibrating element leads to the increase in size of the acceleration sensor, and the reduction in the width W and the thickness T of the vibrating element leads to the reduction in mechanical strength to reduce reliability.